In order to understand the nature of the present invention, it is helpful to first review various exhaust aspects of a turbofan jet engine. Below is a brief description of engine exhaust components and fire safety elements, followed by description of the airflow dynamics associated with these elements during use.
With regard to jet engine exhaust components, FIG. 1 is a forward-looking perspective rear view of a primary exhaust nozzle 20 and a primary nozzle plug 22 of a turbofan jet engine. The primary exhaust nozzle is generally positioned at the aft end of a gas generator (not shown). As illustrated in FIG. 2, the primary exhaust nozzle includes a nozzle outer sleeve 24 concentrically attached to a nozzle inner sleeve 26. The forward end of the nozzle inner sleeve is connected to the gas generator or to adjoining generator structure such as an engine turbine rear frame. An aft engine mount (portions of which are shown in FIG. 2 and labeled 28) is used to connect the engine to a support structure, e.g., an engine strut or pylon.
The support structure of FIG. 1 is a wing strut covered by various fairings and heat shields. Also shown in FIG. 1, and as referred to herein generically as "strut fairings" 30, is an aft cowl fairing 32, an aft cowl fairing heat shield 34, an aft fairing 36, and various heat shield castings 38. An annular core compartment vent exit 42 is formed by the space between the primary nozzle outer sleeve and the thrust reverser aft cowl 39 to vent engine core compartment air. In addition, there may be other outer structures, such as nacelle or thrust reverser components, positioned near the primary nozzle. Shown in FIG. 1 is a thrust reverser inner wall 35, a thrust reverser outer wall 37, a thrust reverser aft cowl 39, and a lower bifurcation panel 40. FIGS. 2-10 omit various of these other structures in order to show aspects and features of the present invention more clearly.
With regard to fire safety elements, a number of fire zones exist within the generator and at the primary nozzle that are designed in a manner that prohibits an engine fire from spreading. Of particular interest to the present invention is an aft mount fire zone located just behind the aft engine mount that extends over the upper 80 degrees of the primary nozzle. Commercial aircraft propulsion systems generally require each fire zone to include a fire seal that is capable of containing and isolating a fire, not only from other propulsion installation components (e.g., nacelles and engine fairings), but also from areas surrounding the propulsion installation (e.g., wings,. fairings, and fuselage). In the case of the aft mount fire zone, a fire seal is located along an upper arcuate region of the nozzle and oriented to prohibit flame from spreading aft of the gas generator or outward to the surrounding structures.
The aft mount fire seal arrangement shown in FIGS. 2, 4, 5, and 8-10 is called a "turkey feather" fire seal 44. Referring to FIG. 4, the turkey feather fire seal consists of a circumferential sheet metal spring 46 formed of segmented steel "fingers" that are overlapped and attached to the primary nozzle outer sleeve forward edge. The turkey feather fire seal compresses against the lower surface of the aft cowl fairing heat shield and the thrust reverser aft cowl (and/or other structure depending on the installation configuration) when the propulsion system is fully installed. The metal spring is angled upward and then downward, in the rearward direction, to provide a solid barrier so that flame cannot pass to downstream locations. This fire seal thus protects the support structures, the strut fairings, the wing structures, the various fairings and shields, and the aircraft fuselage from fire emanating rearward from the gas generator. The fire seal thus forms a barrier across the upper area of the primary nozzle, roughly at the intersection of the nozzle outer sleeve forward edge and the aft cowl faring heat shield.
As shown best in FIG. 4, a region exists on the primary nozzle outer sleeve behind the aft mount fire seal. This region is termed the "batcave" 48. The forward end of the batcave is defined by the aft mount fire seal. Some installations include aerodynamic fairings, termed "batwings" 50 that laterally flank each side of the batcave 48. The aft end of the batcave is not physically bounded.
With regard to fan airflow dynamics, during engine operation the fan air 52 passes generally between the thrust reverser inner and outer walls (thrust reverser stowed), and over the exterior of the primary nozzle as shown in FIG. 3. When the fan air encounters the aft mount fire seal, some of the fan air separates and later converges downstream of the batcave 48. Thus, the fire seal acts as a bluff body.
Because of the geometry of nacelle and strut components and airflow dynamics, there exists differing pressure regions in the areas of the primary nozzle, the aft cowl and aft fairings. In particular, the strut bifurcated fan airflow 52 pressure is relatively high around the batwings, while the pressure in the batcave is relatively low since it is a base region that is somewhat protected from both the fan and primary airstreams 52, 54. In addition, a local high pressure region exists just aft of the primary nozzle exit plane due to primary gas flow impingement along the aft fairing heat shield surfaces.
The combination of the batwing and batcave pressure differences can turn the fan flow laterally. As shown in FIGS. 3 and 4, a portion of fan airflow (labeled 56) may enter the batcave by passing between the aft cowl fairing heat shield and the primary nozzle outer sleeve at the inner side edges of the batwings. This flow converges near the downstream end of the batcave raising the local pressure. The pressure differences can also turn primary airflow forward, into the batcave. A portion of primary exhaust (labeled 58) may enter the batcave by flowing forward through the batcave's unbounded aft end. This batcave pressure field together with the relatively lower pressure at the core compartment vent exit plane induces flow rotation. A steady state representation of the dynamic flow field in the region of the batcave is shown schematically in FIG. 4, the fairing heat shield being shown in phantom. The air entering the batcave has a tendency to enter the batcave sides and aft end, move forward within the batcave, and exit the batcave at its forward lateral corners.
It is the understanding of the present inventors that a phenomenon exists in which side-to-side out-of-phase oscillating pressure fields form and convect axially down the primary nozzle outer sleeve and the aft cowl fairing heat shield exterior surfaces. These pressure fields appear to result from vortex shedding 60 from the airflow coming from the forward lateral corners of the batcave 48. Referring to FIGS. 3 and 4, vortices 60 are formed from a dynamic issuance of the recirculating fan airflow (i.e., the portion entering the batcave lateral sides) and primary airflow (i.e., the portion entering the aft end of the batcave). These airflows exit the batcave with momentum sufficient to couple, amplify, and convect with the surrounding rearward-flowing fan airflow.
The combined batcave recirculation airflows are induced by the pressure fields associated with the gross geometric features of the primary nozzle, outer structures, and fairings, as well as induced by the operating conditions of the jet engine and the aircraft. The batcave vorticity and issuance have a characteristic frequency associated with the local geometry and fan airflow speed. Thus, the phenomenon is tied with the specific component dimensions of the installation, including the nacelle and strut structure geometry.
To the best of the inventors' knowledge, this phenomenon has not been positively identified nor solved prior to the inventors' discovery and solution. The inventors are aware that some other aircraft use flexible seals between the fairing heat shield edges and the primary nozzle outer sleeve in an effort to improve engine performance. Such seals may help prevent the phenomenon described above, but obscure the phenomenon's proper discovery and solution.
Because many current jet engine installations have bounded low pressure regions, such as a batcave, there are potentially many aircraft that would be susceptible to this phenomenon. In general, it is desirable to eliminate flow fluctuations and structural vibrations. Thus a need exists to prevent, eliminate, or at least minimize these fluid disturbances and vortex shedding in propulsion system installations. The present invention is directed to fulfilling this need.